The present invention generally relates to an improved joint prosthesis.
In just over three decades since the inception of its widespread use, total hip replacement (THR) has changed the practice of orthopedic surgery. An entire new industry has been created to support THR and, more importantly, THR has improved quality of life of millions of patients to the extent unparalleled by any other single procedure in the history of surgery. All of this was originally made possible with the advent of cemented THR. Within the first decade following the pioneering work of Charnley, a broad base of trained surgeons, a highly motivated industry and compliant medical insurance policies have led to a world-wide acceptance of THR, evidenced by an exponential growth in the number of procedures being done, currently over 600,000 annually in the world.
This early period of unrestrained optimism was followed, however, by a wave of innovations after the longer-term results became available and the number of revisions (exchange or removal of one or both of the femoral and acetabular components of a THR) started to climb as well. Most of the blame for the increasing revision rate was placed on the bone cement, and efforts to eliminate it, by so-called cementless anchorage, dominated the next decade. But once again, the long term clinical outcomes forced a policy changexe2x80x94cementless THR designs did not match the standard set by the cemented, Charnley-type hip prosthesis design.
In spite of reasonably uniform medical training programs, and widely available and read professional literature, significant differences in attitudes controlling the practice of THR in different geographic regions persist until today and will probably continue to do so. There are countries where over 95% of THR procedures are cemented (e.g. Sweden), but also those where cementless have an edge (e.g. Germany, with about 2 out of 3 THR being cementless). Hybrid THR (i.e., in which one component is cemented and the other is not) has also gained a strong following, especially in the U.S., usually with the acetabular component being cementless and the femoral stem cemented, although the opposite approach has some supporters as well, and the latter is not without merit in view of some recently published clinical outcomes.
The Swedish Hip Registry (and the more recently implemented registries in Norway and in Finland) has been instrumental in weeding out approaches that perform poorly. However, only a limited number of the hip designs currently available worldwide are used in Sweden in numbers large enough to draw any conclusions. Many failures of innovation have also produced a very cautious surgical community, especially in Scandinavia. As the result of this practice of monitoring and control over selection of implants and surgical techniques, imposed by publishing the data from the Registry (including the performance of individual clinics), only one out of ten THR procedures currently done in Sweden is a revision. In comparison, less precise data suggests that one out of five THR procedures in the U.S. is a revision, and the number may be as much as one out of three or four in Germany.
Frequency of revision surgeries depends on many factors other than prosthesis design. However, it is indisputable that one of the primary factors is the still-superior performance of the cemented THR in comparison to cementless. In the Swedish Registry, the overall rate of revisions of cementless THRs is double that of cemented THRs using modem cementing techniques. The debate has come to be dominated by the use of a single outcome parameter of THRxe2x80x94one that the Swedish Registry is based onxe2x80x94the THR xe2x80x9csurvival rate.xe2x80x9d The survival rate of a THR design is defined as the percentage of such THRs which remain in the patient (i.e., which are not revised). Since the survival rate is approximately 95% at ten years for a good prosthesis and a good cement, properly used, it seems that there should be much less pressure for innovation than suggested by the variation of THR models present on the market and by the continuing efforts to improve cementless designs and cementing techniques. One of the reasons spurring new innovation is that both the surgeons and implant manufacturers need something new to gain recognition and market share, which is difficult to do in the non-differentiated field of cemented Charnleys.
Despite the 95% ten-year survival of cemented THRs, it is recognized that there are real problems to solve in the long run. Accordingly, the Swedish Registry has recently undertaken a project to estimate the performance of a smaller number of cases which have not been revised (and thus, until now, were not entering the Registry with an outcome). It is anticipated that the results will show that some 15% of patients have a failed THR at 10 years, but have not been revised for various reasons, such as the advanced age (and limited remaining life expectancy) of the patient. In younger patients revision rates are higher, and cementless THRs currently available seem not to have made any positive impact.
Thus, the limitations of conventional approaches to THR which need to be solved in the long run can be summarized as follows:
in properly-designed cemented stems, most of the failures are due to limited fatigue endurance of the bone cement (leading to so called aseptic loosening);
in cementless stems most of the failures are due to bone loss induced by movement at the stem-bone interface;
in acetabular components, difficulty of matching the implant to highly compliant cancellous bone, with or without cement, leads to formation of soft tissue at the interface which may progress to gross instability;
biological response to wear debris, produced at all interfaces at which relative movement occurs, by intent or accident, including the artificial joint itself, can lead to activation of bone resorption, so called osteolysis, which may progress to gross loosening of prosthetic components, or even to bone fractures.
The invention described herein successfully addresses the challenges of interfacing prosthetic components to bone, allowing the patients unrestricted use of the replaced joint in the immediate post-operative period, yet providing a stable anchorage to the bone of unlimited duration. The invention is disclosed in full detail on an example of the femoral component of a total hip prosthesis, but it is clear that the same principles can be applied to many other prosthesis, e.g. shoulder (humeral component), elbow (both components), knee (both components), finger (both components). It is also applicable to dental and spinal implants. In this disclosure those other applications are only sketchedxe2x80x94one skilled in the art of designing and using such components could certainly, following the example of the total hip prosthesis, produce all necessary implants, instruments and insertion procedures.
The present invention removes a drawback of the screw-fixed femoral components for total hip prosthesis known in the prior art, as exemplified by U.S. Pat. No. 5,458,654 (xe2x80x9cthe ""654 patentxe2x80x9d), namely the necessity to drill access holes in the lateral cortex of the proximal (or upper) femur, and the concomitant need to make incisions in the soft tissue and muscle overlying the lateral cortex. It will of course be appreciated by those of skill in the art that any unnecessary trauma to soft tissue or bone is preferably to be avoided, due to the trauma to the patient""s body, weakening of the femur due to removal of healthy bone matrix, and the chance for infection.
It is one object of the present invention to provide an orthopaedic implant system comprising an orthopaedic implant and at least one bone screw. The orthopaedic implant comprises an intraosseous portion, which intraosseous portion has a first side; a second side; and at least one screw hole. The screw hole has an entry opening at the first side and an internally-threaded portion. The bone screw has an externally-threaded, front end for engagement with the internally-threaded portion of the screw hole, and an externally-threaded head for engagement with the bone. When the front end of this screw is engaged with the internally-threaded portion of the screw hole, the screw is locked to the implant by the interaction of parts of the screw and implant other than the front end of the screw and the internally-threaded portion of the screw hole.
This orthopaedic implant system may be used with a bone having a near cortex, a far cortex, and an intraosseous region; in this case, the externally-threaded head of the bone screw engages with the near cortex of the bone, thereby coupling the orthopaedic implant to the bone in close apposition to (the interior surface of) the near cortex. The screw hole may be either a blind hole, or a through hole which extends to the second side of the intraosseous portion, in the latter case, the front end of the screw may engage with the far cortex of the bone. The bone screw may further be provided with a transition portion, located between the front end and the head, the transition portion provided with self-tapping flutes. The threads of the front end of the screw may have a pitch substantially the same as the threads of the head.
This orthopaedic implant system may include a guide which locks to the implant and has guide holes coaxial with the screw holes in the intraosseous portion when the guide is locked to the implant.
The screw hole may also have a conically tapered portion decreasing in diameter from the entry opening towards the second side. The conically-tapered intermediate portion of the screw preferably has a half-cone angle of about 1 to about 15 degrees, or more preferably of about 3 to about 8 degrees, and most preferably about 5 degrees. The corresponding bone screw will have a conically-tapered intermediate portion tapering from a major diameter near the head to a minor diameter near the front end, with a taper generally matching that of the screw hole.
In this system, it is preferable that the second thread diameter is greater than the major diameter of the intermediate portion and the first thread diameter is less than the major diameter of the intermediate portion. The engagement of the front end of the bone screw with the internally-threaded portion of the screw hole when the screw is inserted into the intraosseous portion draws the conically tapered portion of the screw into engagement with the conically tapered intermediate portion of the screw hole as the screw is advanced, thereby locking the screw and the orthopaedic implant.
It is yet another object of the present invention to provide an embodiment of the implant system which is a hip joint prosthesis. In this embodiment, the implant comprises a femoral component having an intramedullary stem. The intramedullary stem has a medial side, a lateral side, and at least one screw hole. This hip joint prosthesis is suitable for use with a femur having a medial cortex, a lateral cortex, and an intramedullary region, in which case the externally-threaded head of the bone screw engages with the medial cortex, thereby coupling the femoral component to the femur in close apposition to (the interior surface of) the medial cortex.
It is yet another object of the present invention to provide an improved method for implanting an orthopaedic implant in a bone having a near cortex and a far cortex, using screws rather than cement, the implant having an intraosseous portion to be implanted into the patient""s bone. The improvement comprises: inserting the intraosseous portion into the bone of the patient; inserting a bone screw first through the near cortex and then into the implant such that the front end of the screw engages with the intraosseous portion of the implant and the head of the screw engages with the near cortex of the bone; and coupling the orthopaedic implant to the bone in close apposition to (the interior surface of) the near cortex.
This method can also include the steps of: providing an orthopaedic implant having an intraosseous portion having at least one screw-receiving hole; drilling a screw hole in the near cortex of the bone, with no corresponding coaxial hole in the second cortex of the bone, the hole being substantially aligned with the screw-receiving hole in the intraosseous portion; and wherein the step of inserting the bone screw includes inserting the screw through the screw hole in the near cortex and into the screw-receiving hole, such that rotation of the screw advances it in a direction from the near cortex of the patient""s bone towards the far cortex. such that rotation of the screw advances it in a direction from the near cortex of the patient""s bone towards the far cortex and the bone screw engages with the screw hole in the far cortex.
The method can further comprise the step of providing a bone screw having an externally-threaded front end and an externally-threaded head. In this case, when the anchorage screw is inserted into the screw hole in the near cortex and into the screw-receiving hole, the threads of the front end of the screw may engage with the threaded section of the screw-receiving hole before the threads of the head engage the near cortex. The method can further comprise the step of providing a bone screw having a conically-tapered intermediate portion which tapers from a minor diameter towards the front end to a major diameter towards the head, and a transition portion between the intermediate portion and the head, the transition portion being provided with self-cutting flutes. In this case, when the anchorage screw is inserted into the screw hole in the near cortex and into the screw-receiving hole, the threads of the front end of the screw may engage with the threaded section of the screw-receiving hole before the self-cutting flutes of the transition portion engage the near cortex.
The method can further comprise the steps of providing a guide which locks to the prosthesis and has guide holes coaxial with the screw holes in the intraosseous portion when the guide is locked to the prosthesis and using the guide holes in the guide to drill screw holes in the near cortex of the patient""s bone.
It is yet another object of this invention to provide a method, which can advantageously be used for implanting a femoral component of a hip joint prosthesis in the intramedullary canal of the patient""s femur, according to the principles summarized above.
It is yet another object of this invention to provide a femoral component for a cementless hip joint prosthesis, the femoral component comprising an intramedullary stem for insertion into the intramedullary region of a femur, wherein a single size of the intramedullary stem is suitable for use with a wide range of femur sizes. This femoral component can advantageously be used in a method permitting the provision of a femoral component having a stem of one size to fit a wide range of femur sizes.